Computational Fluid Dynamics Reveals a Unique Net Unidirectional Pattern of Pulmonary Airflow in the Savannah Monitor Lizard (Varanus exanthematicus).

Abstract

This report models pulmonary airflow in the savannah monitor (Varanus exanthematicus) using computational fluid dynamics simulations, which are based on computed tomography data. Simulations were validated by visualizing the flow of aerosolized lipids in excised lungs with good but not perfect agreement. The lung of this lizard has numerous successive bronchi branching off a long intrapulmonary bronchus, which are interconnected by intercameral perforations. Unidirectional flow has been documented in the lateral secondary bronchi of the savannah monitor, but patterns of airflow in the rest of the lung remain unknown, hindering our understanding of the evolution of pulmonary patterns of airflow in tetrapods. These results indicate that the lung contains a unique net unidirectional flow, where the overall flow scheme is similar during expiration and late inspiration, but dissimilar during early inspiration. Air is transported net caudally through the intrapulmonary bronchus and net craniad through secondary bronchi, much like the pattern of flow in birds. The simulations show that many chambers feature flow in multiple directions during parts of the respiratory cycle, but some regions also show robust unidirectional airflow. Air moves craniad through secondary bronchi and between adjacent secondary bronchi through intercameral perforations. The first secondary bronchus, the hilar-cranial bronchus, contains tidal flow that may improve ventilation of the central and dorsal lung parenchyma. These results expand our understanding of flow patterns in varanid lungs and suggest lungs with net unidirectional flow as an evolutionary pathway between tidal flow and complete unidirectional flow in multicameral lungs. Anat Rec, 2020. © 2019 American Association for Anatomy Anat Rec, 303:1768-1791, 2020. © 2019 American Association for Anatomy.